Aug. is, 1921 Effect of Ammonium Sulphate upon Plants 711 
culture solutions of the Tottingham series than they are for the corre¬ 
sponding ones of the ammonium-sulphate series. This is brought out 
more clearly by the graphs of figure 1, which represent the average P H 
values plotted in the descending order of their magnitudes in the Totting¬ 
ham series. The relative values of the score for yellowness for the cul¬ 
tures of the Tottingham series and of the ammonium-sulphate series are 
represented graphically by broad and narrow vertical lines, respectively, 
just below the graphs representing the average P H values. 
Comparing now the score for yellowness of leaves with the average P H 
values obtained at the end of the growth intervals, it will be observed 
that the higher P H values throughout the Tottingham series correspond 
in a general way to high relative score values for yellowness in this series. 
The P H values throughout the series decrease slightly with increase in the 
phosphate content of the solutions, and these slight differences are gen¬ 
erally paralleled by somewhat more pronounced differences in the rela¬ 
tive values of the score for yellowness. 
In the cultures of the ammonium-sulphate series the plants were 
entirely free from any chlorotic condition such as characterized the plants 
in the Tottingham series the culture solutions of which had much higher 
average P H values. This is also apparent from the color score, which, 
however, shows a slight degree of yellowness for the plants of several 
cultures only of the ammonium-sulphate series to which reference has 
previously been made. 
Attention has already been called to the fact that the chlorotic con¬ 
dition of the plants in the Tottingham series was due to an insufficient 
supply of available iron, since the addition of soluble iron to the solutions 
of such cultures enabled the plants to regain their normal green color 
and to overcome completely the chlorotic condition in the course of a few 
days. There was no evidence, however, of the lack of available iron in 
the culture solutions of the ammonium-sulphate series. Thus, by the 
substitution of ammonium sulphate for potassium nitrate in the Totting¬ 
ham solutions, the ferric phosphate here used as a source of iron for the 
plants was rendered available perhaps by the maintenance of a higher 
average concentration of the hydrogen ions during the growth of the 
plants in the solutions, since the solubility of iron is, of course, greater 
in solutions of higher hydrogen ion concentrations. 
DRY-WEIGHT YIELDS OF TOPS AND ROOTS 
The relative dry-weight yields of tops and of roots from the cultures 
of the two series here considered are given in Table III. The absolute 
dry-weight yield values corresponding to these relative data have been 
plotted to form the graphs of figures 2 and 3, in which the ordinates 
represent dry weights in grams and the abscissas represent the different 
cultures. The values are always plotted according to the descending 
order of their magnitudes in the Tottingham series. 
